1. Field of the Invention
The present invention relates to a Computed Tomography (CT) system for inspecting baggage for explosives or other contraband. More particularly, it relates to a Computed Tomography (CT) scanning process which does not require complete reconstruction.
2. Discussion of Related Art
Following the terrorist attacks on Sep. 11, 2001, the United States government decided to implement additional airport security. One of the security measures which were to be implemented was inspection of all checked baggage for explosives. In November 2001, Congress passed the Aviation and Transportation Security Act, which mandated 100% explosive detection screening of checked baggage by Dec. 31, 2002. It was expected that the screening requirements would be met through a combination of Explosive Detection Systems (EDS) and Explosive Trace Detection systems (ETD). EDS is faster, but more expensive and more prone to false alarms. ETD is much slower, more invasive and requires more manual input. Congress later extended the deadline for fully deployed until Dec. 21, 2003, on a case by case basis, as long as approved interim methods, such as canine teams, hand searches and passenger bag matching, were employed.
Implementation of this security measure using existing technology has been and will continue to be cumbersome and expensive. Even by the end of 2003, the implemented solutions are unlikely to be permanent solutions. In many cases, airports have deployed hand-fed machines in terminal lobbies, usurping premium space, or implemented the manpower-intensive ETD systems. In order to improve efficiency, reduce manpower requirements, and recover lobby space, airports will transition to EDS machines integrated into airport baggage handling system. However, existing EDS machines are not easily deployed or integrated into existing baggage handling systems. Known Explosives Detection Systems (EDS) utilize either computed Tomography (CT) technology, or a combination of x-ray and CT technology, to create an image of the contents of a bag. Projection x-ray systems have been used for many years with carry-on baggage. However, such systems require operator review of all images and slow throughput. Furthermore, these systems also cannot provide thickness or density information for objects in order to provide explosive detection.
A Computed Tomography (CT) machine has been designed to perform automated explosives detection for passenger baggage, parcels, mail, and small cargo prior to loading onto an aircraft. CT technology has been proven to successfully meet the US TSA Certification requirements for automated explosives detection (EDS) in airline checked baggage. While CT technology is effective for explosive detection, use of existing CT technology in checked baggage inspection has many drawbacks. CT machines incorporate a rotating ring or “gantry” on which the X-ray source and detectors are mounted. FIG. 1 is a cross sectional view of a conventional CT scanner 10. The CT scanner 10 includes a gantry 11 surrounding a tunnel 20. A conveyor (not shown) moves baggage through the tunnel 20 for scanning. The gantry 11 rotates about the tunnel, producing one slice of data for each rotation. An x-ray source 30 produces an narrow angle beam 40. A detector 31 is positioned on the gantry 11 to intersect the x-ray beam 40 passing through the tunnel. The detector 31 may consist of multiple detectors which are located equal distances from the x-ray source. The x-ray source 30 and detector 31 must be sized and positioned so that the entire tunnel falls within the x-ray beam. The data from the detector is analyzed using a computer to generate a three-dimensional representation of the contents of the baggage being scanned.
These CT based systems have many drawbacks. They are large, heavy and require substantial space. The tunnel 20 has to be large enough to accommodate substantially all sizes of checked baggage. The x-source 30 and detector 31 must be positioned sufficient distant from the tunnel 20 so that the entire tunnel is within the x-ray beam 40. Furthermore, the gantry 11 has to be large enough to accommodate the entire detector 31, positioned equidistant from the source. The x-ray source 30 also must be powerful enough to provide sufficient flux at the detector 31, after passing through the baggage, to overcome noise and allow for analysis. The gantry 11 also must be sturdy enough to support and balance the large x-ray source for high-speed rotation. In typical CT systems, the gantry 11 is 5/3 to 2 times the size of the tunnel 20. Furthermore, the system requires significant shielding, generally lead, to protect the operators and passengers from accidental exposure to the powerful x-rays.
The CT scanning and reconstruction process used for baggage inspection is slow and cumbersome. There are two known methods for CT scanning, helical and start/stop. In helical scanning, the object is continuously moved through the scanner. The bag has to be moved slowly so that each rotation of the gantry is substantially in a single plane. In start/stop scanning, the bag is periodically stopped and a single slice is scanned. The bag is then moved a short distance, stopped and scanned again. Both of these processes result in slow movement of baggage through the scanner. Once the data has been collected, the data is reconstructed to create a three dimensional representation of the baggage. From the three dimensional representation, individual items are reviewed as possible threats. The three dimensional representation, or slices of it, may also be displayed for review by an operator.
The operation of CT scanning was originally developed and improved in connection with medical imaging. With medical imaging, resolution is very important. Thus, systems seek to improve the ability to reconstruct images with lots of data, generally with respect to small portions of a body. Scanning speed is less important than accuracy. Additionally, all of the parts of the body need to be analyzed to determine minor differences which can indicate problems. These processes are not readily applicable to baggage scanning. Explosives or other threats are significantly larger than the microscopic items relevant to medical imaging. Furthermore, most objects in a bag are irrelevant to the inspection. However, in prior art system, all objects must be reconstructed prior to the analysis. Baggage inspection systems have addressed these difficulties principally by conducting additional inspections, such as with a flat x-ray, to locate general areas of interest in a bag for CT scanning. Nevertheless, these systems require multiple scanners or multiple passes of baggage through a scanner, which further slows the process.